The present invention relates to liquid, radiation curable compositions, particularly coating compositions, containing an unsaturated polyester component in combination with a vinyl ether component. Such compositions have rapid cure rates initiated by exposure to ultraviolet light or electron beam.
Radiation curable coating compositions based on unsaturated polyester polymers and vinyl ether compounds are the subject of commonly owned, co-pending U.S. Pat. No. 5,536,760 to Charles B. Friedlander and David A. Diehl, which corresponds to European Patent Publication 0 322 808. The use of unsaturated polyester polymers in such compositions, particularly polyesters having relatively high molecular weight, is desirable in that they can produce very good final film properties. However, some unsaturated polyesters that are preferred for their desirable final film properties tend to render the viscosity of the coating formulations unacceptably high for conventional coating application processes.
These coatings also contain vinyl ether compounds of the general formula R--(O--CH.dbd.CH.sub.2).sub.n (where R may be selected from a wide variety of alkyl (groups, and n is at least 1, preferably greater than 1.0). The vinyl ethers are used to produce a three dimensional crosslinked network, and they may also serve to reduce the initial application viscosity of the formulation during coating application. However, the use of the vinyl ether component to reduce viscosity of the coating composition has limits. When formulating such compositions, the stoichiometric ratio of unsaturated bonds in the polyester polymer component to the unsaturated bonds in the vinyl ether component is typically about 1:1. Increasing the vinyl ether component content above the 1:1 stoichiometry in order to reduce viscosity is not an attractive alternative due to the relatively high cost of vinyl ethers and because the excess vinyl ether component may remain uncrosslinked under typical free radical initiated radiation curing conditions. Residual uncrosslinked vinyl ether component remaining after curing may result in a degradation of the final cured film properties.
It is also known to employ reactive diluents (i.e., those that react into the cured film molecular network) such as styrene to reduce the initial viscosity of coating compositions without substantially sacrificing final film properties. Styrene is not desirable as a reactive diluent for radiation curable compositions of the type involved here because styrene slows the cure rate, which is one of the attractive features of these coating compositions.
Solvents are commonly used to reduce the viscosity of coating compositions during application, but the use of solvents is not desirable with the radiation curable coatings of the type involved here. These coatings may be made extremely low in volatile organic content, whereby they are attractive from an environmental standpoint. Using solvents to reduce the viscosity would detract from this desirable feature.
It would be desirable to utilize the advantages of cured film properties yielded by unsaturated polyester polymers in coating compositions having suitable viscosities for application without the disadvantages of substantially higher cost, substantially slower radiation cure rate, or addition of volatile organic components.